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The Gemini North Adaptive Optics (GNAO) facility is the upcoming AO facility for Gemini North providing a state-of-the-art AO system for surveys and time domain science in the era of JWST and Rubin operations. The adaptive optics bench receives both natural star and laser guidestar light from the telescope focus and relays it, via a path that includes a high-rate deformable mirror and a tip-tilt mirror, to science instruments, at 2x magnification. The relay splits off light in a bandwidth of 350 nm - 800 nm for use in three wavefront sensors: a tip/tilt sensor and a low order wavefront sensor using natural guidestar light, and a 4-channel laser guidestar wavefront sensor, supporting a narrow field and a wide field mode (12 arc seconds and 2 arc minutes respectively).
The optical design requirements on the relay are demanding. In particular, exceptionally tight control of distortion was required for the science field, which precluded conventional relays using pairs of off-axis paraboloids. A novel optical system has been produced that meets all of the GNAO AOB requirements. This design is based on an asymmetric version of the Offner relay.
A particularly novel aspect of this relay is that there is no collimated light path within the system, and in this paper it is shown that the system meets all performance requirements without needing such a space. Unlike the normal Offner relay, the “Modified Offner Concentric” (MOC) relay produces an accessible pupil, clear of the confusion of rays at the M2, allowing placement of a flat deformable mirror there.
The MOC design is presented here and is shown to be competitive in this application to conventional AO relays, providing superior performance with reduced complexity.
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